Super-quadratic upconversion luminescence among lanthanide ions.

We explore the arguably most fundamental aspect of energy-transfer upconversion (ETU), namely the dependence of upconversion luminescence from a higher-energy level, following ETU excitation from a metastable lower-energy level, on direct luminescence from that metastable level. We investigate ETU among neighboring Nd ions in single crystals of GdVO and LaSc(BO) with different doping concentrations by measuring, after short-pulse laser excitation with different pump energies, the infrared luminescence decay from the metastable F level and the yellow upconversion luminescence decay from the G level. We observe a highly super-quadratic dependence of upconversion on direct luminescence intensity.